The present invention relates to waterborne film-forming compositions that are curable at ambient or higher temperatures, comprising polyisocyanate crosslinking agents and active hydrogen-containing acrylic copolymers having high glass transition temperatures.
U.S. Pat. No. 5,466,745 to Fiori et al. discloses polyisocyanate functional crosslinking agents which can be emulsified in water with a surface active isocyanate reactive material and the emulsion used to form a curable film-forming composition. The emulsified materials are in the form of an oil-in-water emulsion which exhibits good stability and yet the polyisocyanate and isocyanate reactive material are highly reactive when applied as a film. The patent recommends that these isocyanate reactive materials have glass transition temperature (T.sub.g) less than about 0.degree. C. It is believed the low Tg materials are more mobile during the curing reaction allowing the isocyanate reactive group, i.e., hydroxyl, more opportunity to locate an isocyanate group thereby facilitating the crosslinking reaction. High T.sub.g isocyanate reactive materials, on the other hand, are relatively immobile resulting in less opportunity to locate and react with the polyisocyanate.
A drawback to using low T.sub.g isocyanate reactive materials, however, is that the resultant cured films are inherently softer and less solvent resistant than recommended for automotive applications.
It would be desirable to provide an aqueous based film-forming composition containing a polyisocyanate curing agent and an isocyanate reactive material which can be stably dispersed in water, is reactive when applied as a film, and forms a hard, solvent-resistant film.